The present invention is directed to a monolithically integratable microwave attenuation element which has field effect transistors arranged in a PI structure with terminal ports for a signal path and with control terminals.
Circuits that provide attenuations or gains which are switched between various, discrete values are required in many types of equipment using microwave technology. It is also frequently required of such circuits that a circuit signal experiences only an optimally small phase shift when switching between the individual attenuation or gain values (see, for example, IEEE Transactions on Microwave Theory and Techniques, volume MTT-23, column No. 6, June 1975, pages 461-469).
Also considered in the evaluation of such circuits are the structural size, the obtainable switching speed, the power consumption that is required for switching, the circuit outlay that is needed for driving the switch-over means, the high-frequency bandwidths in which the specified values are valid, the matching of the circuit in the various conditions and the precision with which the various attenuation or gain conditions can be set.
In the prior art, switchable microwave attenuation elements were composed either of bulky mechanical or electro-mechanical switchover devices or of electrically switchable PIN diode attenuation elements (see, for example, IEEE Transactions on Microwave Theory and Techniques, volume MTT-28, column No. 7, July 1980, pages 774-776) or of active elements (field effect transistors) wherein various attenuation or gain conditions could be set by the selection of defined operating points (see, for example, Microwave Journal, Nov. 1982, pages 61-74). Electro-mechanically adjustable attenuation elements are characterized by a large volume, very low switching speeds and high breaking capacities. Although switchable PIN diode attenuation elements usually have an extremely high switching speed, they require high breaking capacities, cannot be monolithically integrated in a simple manner and require relatively large space even in a hybrid structure. Both the mechanical and electro/mechanical attenuation elements as well as the known PIN diode attenuation elements are balanced by direct adjustment of the resistors that are effective in high-frequency terms. As a result the balancing of the elements is complicated.
The setting of defined gain or attenuation values on the basis of the selection of corresponding operating points in active elements can in fact occur relatively fast and with low power. However, a complex matching network is required at the input and at the output of such circuits, this considerably limiting the usable bandwidth and increasing the space requirement. Since the conversion of the control signal into the attenuation condition corresponding to this control signal occurs via the individual characteristic of the active element, a complex balancing is required in such designs. Moreover, changes in gain or attenuation in active elements usually involve a relatively great change of the signal phase.
Previously known circuits having field effect transistors in PI (see, for example, a company publication "Success in Microwaves", published by Plessey Research, (Caswell) limited, England) do not use any resistors connected parallel to the drain-source paths. In arrangements that produce the PI topology only with field effect transistors, the problem of a complicated successive balancing of the field effect transistor control voltages arises given the great unit-dependent tolerances of the channel resistances at a defined control voltage.